This invention relates to the formation of ceramic articles. More particularly, it relates to the formation of porous ceramic articles.
In the production of ceramic materials for use in refractory, kiln furniture, filtration, catalytic, bone implants and other applications, it is sometimes desired to reduce the overall density of the fabricated article by introducing porosity into the article during or after fabrication. The strategy employed for reducing the mass of the article after fabrication usually involves removal of material from the article by means of grinding, drilling, routing or other mechanical methods to physically remove material from selected locations. This usually takes the form of drilling holes, routing channels, etc. Reducing the mass of the material (per unit volume of space occupied by the fabricated article) during fabrication involves using a process which introduces porosity into the ceramic. This can be accomplished by various methods described in the literature.
Some of the basic patents assigned to Selee Corporation, assignee of the present invention, disclose a method to produce a ceramic article with a high volume percent interconnected porosity by impregnating a reticulated polyurethane foam with a ceramic slurry and heating to burn of the polymer and sinter the ceramic. This method can be used to produce various pore sizes and densities. The reported strengths for various ceramic materials fabricated in this manner lie in the 500-700 psi range.
Another method to produce low density ceramic kiln furniture is taught in U.S. Pat. No. 4,812,424, whereby a porous aluminosilicate refractory aggregate is fired. The aluminum metal, alkali silicate and alkali aluminate chemical reaction producing a large volume of small gas bubbles is combined with a sodium silicate-sodium aluminate hydrogel setting reaction which traps the hydrogen gas bubbles in the ceramic. The strengths of this material are approximately in the 500-1000 psi range.
U.S. Pat. Nos. 4,814,300, 4,846,906, 4,871,495, 4,878,947, 4,923,487, 4,963,515 and 4,976,760 are extensions of this basic technology to include membranes and are used in specific markets, such as diesel particulate traps and diesel filters.
European Patent Specification Publication No. EP 0 598 783 B1 discloses a method of preparing porous refractory articles by forming a dispersion comprising particles in a liquid carrier, introducing gas into the dispersion and removing the liquid carrier to provide a solid article having pores derived from the bubbles.
U.S. Pat. No. 4,889,670 discloses a method to produce porous ceramic parts by combining a mixture of 60-90 weight percent of a particulate ceramic with 10-40 weight percent of a latex polymer, whereby the mixture is frothed by mechanical means, shaped, set and sintered to produce the porous article.
It is also well known that porosity can be introduced into a ceramic article by incorporating various types of organic particles into the ceramic body. Upon firing, these particles are oxidized and leave behind voids in the material.